A) Field of the Invention
The present invention relates to an optical semiconductor device and its manufacture method, and more particularly to an optical semiconductor device having a diffraction grating disposed on both sides of a waveguide and its manufacture method.
B) Description of the Related Art
Regrowth structure and vertical diffraction grating structure are known as structures of a distributed feedback (DFB) semiconductor laser device. In the regrowth structure, after a diffraction grating is formed on the surface of a growth layer, a semiconductor layer is regrown thereon. In the vertical diffraction grating structure, after all semiconductor layers are formed, a refraction grating is formed by partially etching the surface layer of the semiconductor layer. As the number of growth of semiconductor layers increases, a manufacture cost rises. It is therefore preferable to adopt the vertical diffraction grating structure for a DFB laser used in a market requiring low cost.
FIG. 10A shows a plan cross sectional view of a DFB laser device of the vertical diffraction grating structure, and FIG. 10B shows a cross sectional view taken along one-dot chain line 10B-10B shown in FIG. 10A. A cross sectional view taken along one-dot chain line 10A-10A shown in FIG. 10B corresponds to FIG. 10A. The DFB laser device is disclosed, for example, in JP-A-HEI-8-167759.
As shown in FIG. 10A, a waveguide region 101 is defined in the surface layer of a semiconductor substrate 100, the waveguide region 101 extending from one facet to the other of a pair of facets facing mutually opposite directions. A diffraction grating 102 is disposed on both sides of the waveguide region 101. An antireflection film 103 or a high reflection film 104 is formed on each of the pair of facets.
As shown in FIG. 10B, a lower cladding layer 110, a waveguide layer 111, an active layer 112, an upper cladding layer 113 and a contact layer 114 are laminated in this order on the semiconductor substrate 100. The diffraction grating 102 is constituted of low refractive index material filling a plurality of recesses reaching an intermediate depth of the upper cladding layer 113. An upper electrode 115 is formed on the contact layer 114, and a lower electrode 116 is formed on the bottom of the semiconductor substrate 100.
In the DFB laser device shown in FIGS. 10A and 10B, the opposite facets of the waveguide region 101 are formed, for example, by cleaving the substrate 100. Since cracks are likely formed while the substrate is cleaved, it is difficult to form facets at target positions. For example, a facet is formed in some cases at a position shifted from the target position by one period of the diffraction grating. Positions of the facet of one diffraction grating and the facet of the other diffraction grating are not in alignment with each other in some cases.
JP-A-2005-353761 discloses a DFB laser device of the regrowth structure not disposing a diffraction grating near the facets. This structure may be applied to a DFB laser device of the vertical diffraction grating structure.
FIG. 10C shows a plan cross sectional view of a DFB laser device of the vertical diffraction grating structure adopting the structure disclosed in JP-A-2005-353761. Regions 118 not disposing a diffraction grating 102 are provided near the facets of the DFB laser device. Since the diffraction grating 102 of this DFB laser device is positioned away from the cleaved facets, cracks are hard to be formed during cleavage.
In a DFB laser device of the regrowth structure, a ridge defining a waveguide extends from one facet to the other of the DFB laser device. Therefore, optical confinement in the width direction is realized in the whole region of the waveguide. In a DFB laser device of the vertical diffraction grating structure, however, light is confined in the width direction by the diffraction gratings 102 shown in FIGS. 10A and 10C. Since the diffraction gratings are not formed in the regions 118 near the facets of the DFB laser device, optical confinement in the width direction is not realized, but optical confinement is realized only in the thickness direction in these regions 118. Namely, the regions 118 function as a slab waveguide.
Therefore, a transverse mode shape of light confined in the region 118 not disposing the diffraction grating changes from a desired shape. A coupling efficiency between a DFB laser device and an optical fiber is therefore lowered.